This invention relates to an improvement of a display apparatus.
With the increase in capacity and decrease in costs of IC memories in recent years, raster scanning color graphic display systems having a large capacity bit map memory plane are becoming more compact and less expensive. Such a color graphic display comprises three bit map memory planes respectively corresponding to three primary colors, i.e., red, green and blue, a function generator and a microprocessor. When displaying a graphic pattern in the graphic color display system, the function generator is made to generate display information, which is successively developed into the respective memory planes and is then cyclically read in synchronism with the raster scanning, and is transferred to a CRT monitor to result in the desired display.
During the display, logical or arithmetic operations are frequently conducted on the contents read out of the memory locations of the memory planes of the same or corresponding picture elements.
For instance, in a color display apparatus, there may be provided three memory planes for storing data of the three primary colors, and it may be desired to form an image pattern consisting of the area on which two or more colors are superimposed or mixed, or to select the area of a single specific color. In such cases, it is necessary to perform logical operations on the data from the memory planes.
To meet the need of logical operations, a conventional picture display apparatus generally employs a high-speed microprocessor which sequentially reads the data from the memory planes and performs logical operations on the data read out.
Such an arrangement is satisfactory small capacity memory planes. But with memory planes of a large capacity, e.g., 1024.times.1024 dots, it takes a long time for the microprocessor to read the data from the memory planes and to conduct the logical operations on the data read out, so that the performance of the overall device is limited.